1. Field of the Invention
The present invention relates to a channel characteristics estimation system, a channel characteristics estimation method, a communication apparatus, and a communication method for estimating channel characteristics under wireless communication environment and particularly to a channel characteristics estimation system, a channel characteristics estimation method, a communication apparatus, and a communication method for estimating channel characteristics in order to more accurately measure the arrival time of reception signals for the purpose of ranging.
More specifically, the present invention relates to a channel characteristics estimation system, a channel characteristics estimation method, a communication apparatus, and a communication method for estimating channel characteristics in wide bands and particularly to a channel characteristics estimation system, a channel characteristics estimation method, a communication apparatus, and a communication method for estimating channel characteristics in wider bands in a wireless communication system that performs frequency hopping.
2. Description of Related Art
Special attention is paid to wireless LAN as a system to free users from cabling of hardwired LANs. The wireless LAN can eliminate most of cables in working spaces such as offices. Accordingly, it is possible to relatively easily move communication terminals such as personal computers (PCs). In recent years, there is a remarkably increasing demand for wireless LAN systems as they achieve higher speed and become available at reduced costs. Recently, introduction of a personal area network (PAN) is especially being considered to construct a small-scale network for information communication between electronic devices available around users. For example, there are provided different wireless communication systems and wireless communication apparatuses using frequency bands such as 2.4 GHz and 5 GHz bands that need not be licensed by governing legal authorities.
In recent years, for example, attention is focused on the “ultra wide band (UWB) communication” as a wireless communication system capable of short-distance, ultrafast transmission. The system performs wireless communication by carrying information on very weak impulse sequences. It is expected to put the system into practical use. Presently in IEEE802.15.3 and the like, there are devised data transmission systems having the packet structure including preambles as access control systems for ultra wide band communication.
If a wireless network is constructed under a working environment where many devices are mixed in a room, it is possible to suppose that a plurality of networks are constructed in an overlapping fashion. A wireless network using a single channel cannot provide any countermeasure against a case where another system interrupts during communication or an interference occurs to degrade the communication quality. To solve this problem, there is proposed a multi-channel communication system that provides a plurality of frequency channels and performs frequency hopping to operate. If an interference occurs to degrade the communication quality during communication, for example, frequency hopping is used to maintain network operations, enabling coexistence with the other networks.
When a wireless network is constructed in a room, receivers form a multipath environment to receive an overlap of a direct wave and a plurality of reflected waves or delay waves. Multipath generates a delay distortion (or frequency selective fading) to cause a communication error. Further, a delay distortion causes inter symbol interference.
A major countermeasure against delay distortion can be a multi-carrier transmission system. The multi-carrier transmission system transmits data by dividing it into a plurality of carriers having different frequencies. Each carrier uses a narrow band and is hardly subject to frequency selective fading.
For example, the OFDM (Orthogonal Frequency Division Multiplexing) system, one of multi-carrier transmission systems, configures a frequency of each carrier so that the carriers become orthogonal to each other in a symbol region. During information transmission, the system converts serially transmitted information into parallel information at a symbol frequency lower than the information transmission rate. The system allocates a plurality of pieces of output data to each carrier, modulates the amplitude and the phase for each carrier, and performs the inverse FFT for the carriers. In this manner, the system converts the carriers into signals along the time-domain by maintaining the orthogonality of each carrier along the frequency-domain. The reception occurs in the reverse order of the transmission. The system performs the FFT to convert signals along the time-domain into those along the frequency-domain and demodulates the carriers in accordance with the modulation of each carrier. The system performs parallel-serial conversion to reproduce the information that was originally transmitted in the serial signals.
The OFDM modulation system is adopted as a wireless LAN standard in the IEEE802.11a/g, for example. The IEEE802.15.3 standardization is also in progress for the UWB communication system using the OFDM modulation system in addition to the DS-UWB system and the impulse-UWB system. The DS-UWB system increases spread speeds of DS information signals to the utmost limit. The impulse-UWB system uses impulse signal sequences having very short frequencies of several hundred picoseconds to configure information signals for transmission and reception. The OFDM_UWB communication system investigates an OFDM modulation that performs frequency hopping (FH) for a frequency band of 3.1 through 4.8 GHz into three sub-bands each comprising 528 MHz bandwidths and uses IFFT/FFT with frequency band comprising 128 points (e.g., see non-patent document 1).
On the other hand, the UWB communication uses ultra narrow pulses to provide high time resolution. This property can be applied to ranging for radar and positioning. In particular, the latest UWB communication can provide both high-speed data transmission over 100 Mbps and the intrinsic ranging function at the same time (e.g., see patent document 1).
In the future, it is expected that WPAN (Wireless Personal Access Network) for near field communication represented by the UWB communication is installed in all household electrical goods and CE (Consumer Electronics) devices. Therefore, in addition to the high-speed data transmission, it is considered to use position information based on the ranging, e.g., provide wireless added values such as navigation and Near Field Communication (NFC). It may be desirable to provide not only the high-speed data transmission, but also the ranging function.
For example, the UWB communication standardization in IEEE802.15.3a includes the UWB ranging technology (e.g., see non-patent document 1).
It is a general practice to start ranging from time T from transmission of a packet to reception thereof. In order to improve the ranging resolution, it is important to measure channel characteristics under multipath environment in as wide a band as possible in a short time period and to more accurately measure arrival time T of a reception signal. This is equivalent to a fine pulse width. If the measurement can be performed in units of nanoseconds, for example, the ranging is available at resolution of approximately 30 cm.
The following describes the ranging resolution in the UWB communication system that is being standardized in IEEE802.15.3a. As mentioned above, it is highly possible that the communication system will adopt a frequency hopping system sub-banded every 528 MHz. Performing the ranging for each sub-band just provides a 2-nanosecond time resolution (approximately 60 cm as a spatial resolution) equivalent to the reciprocal number of the bandwidth. The ranging accuracy becomes insufficient.
If it is possible to integrate channel characteristics estimated in each sub-band where frequency hopping is performed, wide-band channel characteristic estimation is feasible. However, the following problem arises when an orthogonal frequency modulation system such as OFDM performs the wide-band channel characteristics estimation using all bands.
FIG. 9 schematically shows the mechanism of modulation, propagation, and demodulation in the OFDM modulation system. FIG. 9 is viewed from the time region.
First, transmission baseband signal x(t) is multiplied (1) by complex sine wave rot(fct) of carrier frequency fc (2) to convert the frequency (3). In this example, function rot(x) is defined to be exp(2πjx).
A transmission RF signal can be obtained by taking the real part of the complex sine wave rot(fct) (4). An impulse response called channel characteristics h(t) passes through the channel (5).
The reception side multiplies complex sine wave rot(−fct) whose positive and negative signs are in inverse relation to the counterparts of the transmission side, i.e., takes the complex conjugate of the complex sine wave of the transmission side to convert the frequency (7). The result is allowed to pass through a low-pass filter LPF (8) to obtain a reception baseband signal (8).
FIG. 10 shows the orthogonal modulation and demodulation mechanism in FIG. 9 viewed from a frequency region;
That is to say, transmission baseband signal X(f) (1) is convoluted with complex sine wave δ (f−fc) of carrier frequency fc (2) to convert the frequency (3).
The transmission RF signal is obtained by generating a complex-conjugate-symmetric component against frequency 0 (4). The channel allows frequency characteristic H(f) to pass (5).
The reception side convolutes the frequency's complex sine wave δ (f+fc) whose positive and negative signs are in inverse relation to the counterparts of the transmission side (6), i.e., converts the frequency (7). The result is allowed to pass through a low-pass filter LPF to obtain reception baseband signal H(f−fc)X(f) (8).
Let us assume that the transmission baseband signal X(f) is a known training signal between transmission and reception sides, for example. The reception side can divide the reception baseband signal H (f−fc)X(f) by the training signal component X(f) to obtain channel characteristics H(hat) (f−fc) in fc at which the signal is propagated.Ĥ(f−fc)=H(f−fc)X(f)/X(f)=H(f−fc)  [Equation 1]
The orthogonal modulation and demodulation system in FIG. 9 gives no consideration to a phase difference between frequencies used for the frequency conversion. Actually, both the transmission and reception sides each use a high precision temperature compensated crystal oscillator (TCXO) of approximately 1 ppm to obtain frequencies with errors small enough for the frequency conversion. However, it is impossible to make adjustment as accurately as the phase of complex sine waves. Let us define φT and φR to be complex sine wave's phase differences in the transmission and reception sides, respectively. Then, the orthogonal modulation and demodulation system in FIG. 9 is modified as shown in FIG. 11.
When considering the complex sine wave's phase difference φT at the transmission side, transmission baseband signal x(t) (1) is multiplied by complex sine wave rot(fct+φT) of carrier frequency fc containing phase difference φT (2) to convert the frequency (3). The transmission RF signal can be obtained by taking the real part of complex sine wave rot(fct+φT) (4). An impulse response called channel characteristics h(t) passes through the channel (5).
The reception side multiplies the impulse response by complex sine wave rot(−fct−φR) (6) that has positive and negative signs in inverse relation to the counterparts of the transmission side and contains phase difference φR to convert the frequency (7). The result is allowed to pass through the low-pass filter LPF (8) to obtain a reception baseband signal (8). From the viewpoint of the frequency region, there is provided reception baseband signal rot(φT−φR) H (f−fc)X(f) containing phase rotation component rot(φT−φR) based on phase differences φT and φR in the transmission and reception sides, respectively. Therefore, a channel characteristics estimation value contains this phase rotation component rot(φT−φR) as shown in the following equation.Ĥ(f−fc)=rot(θ)H(f−fc)X(f)/X(f)=rot(φT−φR)H(f−fc)  [Equation 2]
The phase rotation component rot(φT−φR) corresponds to phase rotation of the entire signal and causes no particular problem only if the single frequency channel fc is used for transmission and reception. On the contrary, this specification assumes the frequency hopping system that performs hopping between a plurality of sub-bands. Further, the system aims at integrating channel characteristics estimation values in each sub-band to provide wide-band channel characteristics estimation. This makes clear the problem of phase differences φT and φR contained in the transmission and reception sides. This topic will be described in more detail.
As shown in FIG. 12, the frequency hopping system can provide wide-band transmission by sequentially changing frequencies f1, f2, and f3 to be transmitted for each time slot.
FIG. 13 illustrates a mechanism to perform the orthogonal modulation and demodulation for each frequency channel in the frequency hopping system in FIG. 12. In this example, it is assumed that phase differences φT1, φT2, and φT3 are contained corresponding to the transmission frequencies f, f2, and f3 in a complex sine wave used for the frequency conversion at the transmission side. It is also assumed that phase differences φR1, φR2, and φR3 are contained in a complex sine wave used for the frequency conversion at the reception side.
Transmission baseband signals x(1) (t), x(2) (t), and x(3) (t) (1) corresponding to the transmission frequencies are multiplied by complex sine waves rot(f1t+φT1), rot(f2t+φT2), and rot(f3t+φT3) corresponding to the carrier frequencies containing the phase differences φT1, φT2, and φT3 (2) to convert the frequencies (3). The transmission RF signal can be obtained by taking the real part of each complex sine wave (4). An impulse response called channel characteristics h (t) passes through the channel (5).
The reception side multiplies complex sine waves rot(−f1t−φR1), rot(−f2t−φR2), and rot(−f3t−φR3) that have positive and negative signs in inverse relation to the counterparts of the transmission side and contain phase differences φR1, φR2, φR3 corresponding to the transmission frequencies to convert the frequencies (7). The result is allowed to pass through the low-pass filter LPF (8) to obtain a reception baseband signal (8).
From the viewpoint of the frequency region, there are provided reception baseband signals rot(φT1−φR1) H (f−f1) X(1) (f), rot(φT2−φR2) H (f−f2) X(2) (f), and rot(φT3−φR3) H (f−f3) X(3) (f) containing phase rotation components based on the phase differences in the transmission and reception sides corresponding to the transmission frequencies. Consequently, channel characteristics estimation values are obtained for the corresponding transmission frequencies and inevitably contain different phase rotation components rot(φT1−φR1), rot(φT2−φR2), and rot(φT3−φR3) as shown in the following equation.Ĥ(f−f1)=rot(φT1−φR1)H(f−f1)Ĥ(f−f2)=rot(φTs−φR2)H(f−f2)Ĥ(f−f3)=rot(φT3−φRs)H(f−f3)  [Equation 3]
That is to say, it is possible to measure frequency characteristics in each band (see FIG. 14). However, the channel characteristics estimation values obtained for the transmission frequencies are not continuous as complex numbers (discontinuous phases). It is impossible to join these values so as to be assumed to be continuous frequency characteristics. FIG. 15 shows amplitudes and phase arg (H (hat) (f)) of channel characteristics estimation value | H(hat) (f) | in each frequency hopping. As can be seen from FIG. 15, the amplitudes are continuous in the frequency hoppings. However, the phases are discontinuous because they deviate from desired values due to phase differences contained in the frequencies. In other words, it is impossible to integrate channel characteristics estimation values in the sub-bands for hopping so as to generate a wide-band channel characteristics estimation value.
When ranging is performed in the communication systems such as UWB, using a higher time resolution to detect time responses on the channel is equivalent to using a higher distance resolution to measure a distance. When the transmission estimation is required in the frequency region, it is necessary to find continuous frequency characteristics in wider bands.
[Patent document 1] Japanese Translation of Unexamined PCT Appln. 2002-517001
[Non-patent document 1] IEEE802.15.3a TI Document <URL: http://grouper.ieee.org/groups/802/15/pub/2003/May03 filename: 03142r2P802-15_TI-CFP-Document.doc>